Once a diamond operation yields ore, the diamonds must be sorted from the other materials. Excavated ore is transported to a processing plant.
The kimberlite is first crushed and then processed through the plant, which consists of a series of screens, jigs and scrubbers and a gravity pan or DMS plant to remove lighter particles and create a concentrate of heavy material, which includes the diamonds.
Diamonds are then extracted from this material by using an X-ray machine and/or grease table and checked by hand sorting. Most diamonds luminesce under X-rays and can therefore be identified and separated in final recovery. However, some diamonds – particularly more valuable Type II stones – do not respond well to X-rays, so grease tables are used to recover such stones. As diamonds are hydrophobic (meaning they repel water), they stick to the grease while the rest of the wet concentrate runs off.
The treatment of ROM kimberlite ore produces a sink (high density material in which diamonds are concentrated) and a float product. The float material (which may still contain diamonds) is discarded onto a coarse tailings dump. Once the sink material has been further treated and examined for diamonds it is also discarded onto a recovery tailings dump. The discarded material can then be mined and treated at a later stage if the diamond price renders it economically feasible.